Alkaline pulping processes use a white liquor containing sodium hydroxide and sodium sulfide to extract fibers from wood chips in a high temperature pressurized digester. The black liquor discharged from the digester is evaporated and combusted. The black liquor combustion residue is then dissolved to form green liquor which is thereafter converted back to the white liquor through a lime cycle. The lime cycle typically includes a slaker in which the green liquor is causticized by lime to form a precipitated calcium carbonate mud, and a lime kiln in which the calcium carbonate mud is reconverted to lime by calcining.
In the above process, lime can be partially or entirely replaced by sodium borate. Sodium borate circulating through the plant process can automatically causticize the plant liquors by reacting with sodium carbonate to produce a trisodium borate (Na3BO3) which then reacts with water to generate sodium hydroxide. The use of borate could reduce or eliminate the cost of the lime cycle including equipment, energy and operation expenses. See, for example, U.S. Pat. No. 4,116,759 to Janson (the entire content of which is expressly incorporated hereinto by reference).
Measurement of white liquor alkali concentrations (hydroxide, sulfide and carbonate) is important to produce a uniform pulp quality and for stable digester operation. Determination of green liquor alkali concentrations is useful in causticizing control. Analysis of borate in both white and green liquors is necessary to control the amount of sodium borate needed to achieve the targeted autocausticizing effect. Therefore the ability to conduct these analyses efficiently and reliably plays a crucial role in promoting this new application.
The pulping industry has traditionally used an acid titration method, commonly referred to as the ABC method, to analyze hydroxide, carbonate and sulfide. See in this regard, Standard J.12, “Analysis of Sulphate Green and White Liquors”, Canadian Pulp & Paper Association, (June, 1961), the entire content of which is expressly incorporated hereinto by reference. Generally, the ABC method involves a manual acid-base titration procedure with three equivalence points at different pH's. The equivalence point A (pH=10-11) detects hydroxide plus one-half sulfide; equivalence point B (pH=8-10) detects another one-half sulfide; and equivalence point C (pH=4.014 5.5) detects carbonate. It is a widely accepted and simple procedure which can be carried out manually, using pH sensitive color indicators. However, the presence of borate interferes with the conventional ABC method because borate is an effective buffer at a pH of 9 and thus obscures the equivalence point B.
Pulping liquors without borate can also be analyzed using an acid-base titration procedure on an autotitration system, again producing three pH equivalence points. In this case, equivalence point A (pH=10-11) detects hydroxide plus one-half sulfide; equivalence point B (pH=7.5-8.5) detects one-half of carbonate; and equivalence point C (pH=4.0-5.5) detects another one-half of sulfide and another one-half of carbonate. However, the presence of borate also interferes with this procedure because borate and carbonate have inseparable equivalence points at a pH in the range of 7.5 to 8.5. This makes the titration results indeterminate for all species because of the multi-step reactions and the interrelationship of the titration endpoints between the various species involved.
It would therefore be highly desirable if analytical methods could be provided which enable chemical species to be analyzed in boron-containing alkaline pulping liquors. It is towards fulfilling such a need that the present invention is directed.
Broadly, therefore, the present invention is embodied in methods by which borate-containing pulping liquors can be analyzed. The methods of the present invention therefore overcomes many (if not all) of the problems associated with analyzing borate-containing liquors by conventional techniques. Most preferably, the analysis methods of the present invention are automated for increased simplicity and reliability.
More particularly, the present invention provides an improved method for analyzing boron-containing alkaline wood pulping liquors for the determination of sulfide, hydroxide, carbonate and boron, as well as other useful solution properties which are dependent on these chemical analyses and are important to the operation of a pulping process. The method of the present invention most preferably comprises use of the acid titration procedure in concert with the determination of either boron or sulfide by other analytical methods and integration of the resulting data through a series of equations to determine the sulfide, hydroxide, carbonate and boron content of the solution. Preferably, the acid titration procedure is carried out on an automatic titration system. Most preferably sulfide is measured by precipitation as silver sulfide, using a second autotitration unit, and the two autotitration units are coupled using a programmable unit such that the acid and sulfide analyses can carried out automatically and the results can be automatically input and analyzed using a series of equations, such that the desired results are provided automatically by the instrument.
In especially preferred embodiments of the present invention, chemical species (e.g., metaborate, carbonate, hydroxide and sulfide) in a boron-containing alkaline wood pulping liquor sample are determined quantitatively by (i) subjecting a first aliquot portion of the sample to a primary acid titration analysis to derive multiple equivalence points at different respective pH values; (ii) subjecting a second aliquot portion of the sample to an analysis to determine the quantitative presence of boron or sulfide ions therein, and then (iii) determining the quantitative presence in the sample of at least one of the chemical species. Wood pulping parameters may thus be determined on the basis of the quantitative presence of the chemical species to assist in process and/or quality control of the wood pulping operation. Alternatively, wood pulping parameters may be derived based on the analytical results of steps (i) and (ii), without determining the quantitative presence of the chemical species.
For example, the sample may be analyzed for boron content using various techniques, such as, for example, colorimetry or atomic spectroscopy (e.g., flame atomic adsorption (FAA) or inductively coupled plasma (ICP)) and/or analyzed for sulfide ion content using secondary silver sulfide precipitation titration analysis. These techniques may be conducted substantially concurrently with the primary acid titration analysis. Most preferably sulfide ion content is analyzed using a secondary silver sulfide precipitation analysis and the primary acid titration analysis and secondary silver sulfide precipitation analysis are conducted substantially simultaneously in parallel on separate independent autotitration units which are electronically coupled and preprogrammed such that all necessary analytical results and solution properties are calculated and reported automatically.
These and other aspects and advantages will become more apparent after careful consideration is given to the following detailed description of the preferred exemplary embodiments thereof.